1. Field of the Invention
The present invention relates to a base for a magnetic recording medium.
2. Description of the Related Art
There are demands to considerably improve a recording density (or storage capacity) of a magnetic recording medium that is used in a hard disk drive. Particularly, introduction of a MR (Magneto-Resistive) head and the PRML (Partial Response Maximum Likelihood) technique has further improved an in-plane recording density of the magnetic recording medium.
On the other hand, due to recent developments in areas of the Internet and big data utilization, a data storage capacity of a data center continues to increase. Due to space limitations at the data center, there are demands to increase the storage capacity per unit volume. In other words, in order to increase the storage capacity of one standardized hard disk drive, there are proposals to increase a number of magnetic recording media accommodated within a casing of a standardized hard disk drive.
An Al alloy substrate or a glass substrate is primarily used as the base for the magnetic recording medium. Compared to the glass substrate, the Al alloy substrate has a higher toughness and more manufacturing ease, and thus, the Al alloy substrate is used for the magnetic recording medium having a relatively large diameter. A thickness of the Al alloy substrate used for the magnetic recording medium of a 3.5-inch standardized hard disk drive is 1.27 mm, for example.
However, when the thickness of the base used for the magnetic recording medium is reduced, fluttering more easily occurs in the case in which the Al alloy substrate is used when compared to the case in which the glass substrate is used. Fluttering of the magnetic recording medium occurs when the magnetic recording medium is rotated at a high speed. When the fluttering occurs, it becomes difficult to stably read information from the magnetic recording medium within the hard disk drive. In the case in which the glass substrate is used, for example, the base for the magnetic recording medium is made of a material having a high Young's modulus in order to suppress the fluttering, as proposed in Japanese Laid-Open Patent Publication No. 2015-26414, for example.
Generally, the Al alloy substrate is manufactured as follows. First, an Al alloy plate having a thickness on the order of 2 mm or less is formed into a donut-shaped substrate having a desired size by punching. Next, inner and outer peripheral edges of the donut-shaped substrate are chamfered, and data recording surfaces of the donut-shaped substrate are subjected to machining by rotation. Surface roughness and undulation of the data recording surfaces are reduced by grinding using a honing stone. Thereafter, from a viewpoint of providing surface hardness and suppressing surface defects, the substrate surfaces are NiP-plated. Next, both substrate surfaces (or data recording surfaces), having a NiP-plated film, are subjected to polishing. The base for the magnetic recording medium is a mass-produced product, and a high cost performance is desired of the base. For this reason, a high machinability and a low cost are desired of the Al alloy used for the base.
For example, Japanese Laid-Open Patent Publication No. 2009-24265 proposes a material having a high machinability and capable of suppressing damage such as frictional wear, chipping, or the like to a cutting tool. The proposed material is an Al alloy having a satisfactory anodizable property, and includes 0.3 mass % to 6 mass % of Mg, 0.3 mass % to 10 mass % of Si, 0.05 mass % to 1 mass % of Zn, and 0.001 mass % to 0.3 mass % of Sr, with the remainder being Al and impurities.
Accordingly, it is difficult to reduce a thickness of the base for the magnetic recording medium and obtain a high Young's modulus.